1. Technical Field
The present invention is concerned with speech recognition, particularly although not exclusively for use in automated voice-interactive services for use over a telephone network.
2. Related Art
A typical application is an enquiry service where a user is asked a number of questions in order to elicit replies which, after recognition by a speech recogniser, permit access to one or more desired entries in an information bank. An example of this is a directory enquiry system in which a user, requiring the telephone number of a customer, is asked to give the town name and road name of the subscriber's address, and the customer's surname.
The problem with a system which is required to operate for a large number of customer entries, the whole of the UK which has about 500 thousand different surnames, for example, is that once the surname vocabulary becomes very large the recognition accuracy falls considerably. Additionally the amount of memory and processing power required to perform such a task in real time becomes prohibitive.
One way of overcoming this problem is described in our co-pending patent application EP 95934749.3 in which,                (i) the user speaks the name of a town;        (ii) a speech recogniser, by reference to stored town data identifies several towns as having the closest matches to the spoken town name, and produces a “score” or probability indicating the closeness of the match;        (iii) a list is compiled of all road names occurring in the identified towns;        (iv) the user speaks the name of a road;        (v) the speech recogniser identifies several road names, of the ones in the list, having the closest matches to the spoken road name, again with scores;        (vi) the road scores are each weighted accordingly to the score obtained for the town the road is located in, and the most likely “road” result considered to be the one with the best weighted score.        
A disadvantage of such a system is that if the correct town is not identified as being one of the closest matches then the enquiry is bound to result in failure.